Working off the Specs here .... http://webcache.googleusercontent.com/search?q=cache:awpCnJhURMoJ:www.voodoolab.com/pedalpower_2.htm+voodoo+lab+pedal+power+2+plus&cd=1&hl=en&ct=clnk&gl=uk&client=firefox-a (main site was down !).

It looks like the max voltage and current that the Pedalpower can supply is  9V, 250mA.

The Standard Line6 DC3-G power supply puts out 9V, 3000mA   (i.e. 3 amps).  That means that the Line 6 PSU can supply up to twelve times more current than the Pedal Power.  

With that level of mismatch, I think there is very little chance that you can use the Pedal Power safely with the HD500 series.  You actually run a fire risk if there is inadequte protection within the Pedal Power.

I wouldn't.